CLEVELAND — Declaring America in crisis, Donald Trump pledged to cheering Republicans and still-skeptical voters Thursday night that as president he will restore the safety they fear they're losing, strictly curb immigration and save the nation from Hillary Clinton's record of "death, destruction, terrorism and weakness."

Donald Trump speaks on the last day of the Republican National Convention, July 21. (Photo: Jim Watson/AFP/Getty Images)

Confidently addressing the finale of his party's less-than-smooth national convention, the billionaire businessman declared the nation's problems too staggering to be fixed within the confines of traditional politics.

"I have joined the political arena so that the powerful can no longer beat up on people that cannot defend themselves," Trump said.

The 70-year-old celebrity businessman's acceptance of the Republican nomination caps his improbable takeover of the GOP, a party that plunges into the general election united in opposition to Clinton but still divided over Trump. Underscoring his unorthodox candidacy, Trump doubled down on the hard-line immigration policies that fired up conservatives in the primary but broke with many in his party by promising protections for gays and lesbians.

His address on the closing night of the convention marked his highest-profile opportunity yet to heal Republican divisions and show voters he's prepared for the presidency. Ever the showman, he fed off the energy of the crowd, stepping back to soak in applause and joining the delegates as they chanted, "U-S-A."

"I'm with you, and I will fight for you, and I will win for you."

As the crowd, fiercely opposed to Clinton, broke out in its oft-used refrain of "Lock her up," he waved them off, and instead declared, "Let's defeat her in November." Yet he also accused her of "terrible, terrible crimes" and said her greatest achievement may have been avoiding prison for her use of a private email and personal server as secretary of state.

The more than hour-long speech was strikingly dark for a celebratory event and almost entirely lacking in specific policy details. Trump shouted throughout as he read off a teleprompter, showing few flashes of humour or even a smile.

He accused Clinton, his far-more-experienced Democratic rival, of utterly lacking the good judgment to serve in the White House and as the military's commander in chief.

"This is the legacy of Hillary Clinton: death, destruction, terrorism and weakness," he said. "But Hillary Clinton's legacy does not have to be America's legacy."

In a direct appeal to Americans shaken by a summer of violence at home and around the world, Trump promised that if he takes office in January, "safety will be restored."

Trump addresses delegates on the final night of the Republican National Convention (Photo: Timothy A. Clary/AFP/Getty Images)

As he moves into the general election campaign, he's sticking to the controversial proposals of his primary campaign, including building a wall along the entire U.S.-Mexico border and suspending immigration from nations "compromised by terrorism."

But in a nod to a broader swath of Americans, he said young people in predominantly black cities "have as much of a right to live out their dreams as any other child in America." He also vowed to protect gays and lesbians from violence and oppression, a pledge that was greeted with applause from the crowd.

"As a Republican, it is so nice to hear you cheering for what I just said," he responded.

Trump was introduced by his daughter Ivanka, who announced a childcare policy proposal that the campaign had not mentioned before.

"As president, my father will change the labour laws that were put in place at a time when women weren't a significant portion of the workplace, and he will focus on making quality childcare affordable and accessible for all," she said.

Ivanka Trump speaks during the final day of the Republican National Convention, July 21, 2016. (Photo: J. Scott Applewhite/The Associated Press)

Trump took the stage in Cleveland facing a daunting array of challenges, many of his own making. Though he vanquished 16 primary rivals, he's viewed with unprecedented negativity by the broader electorate, and is struggling in particular with younger voters and minorities, groups GOP leaders know they need for the party to grow.

The first three days of this week's convention bordered on chaos, starting with a plagiarism charge involving his wife Melania Trump's speech and moving on to Texas Sen. Ted Cruz's dramatic refusal to endorse him from the convention stage.

Then, Trump sparked more questions about his Oval Office readiness by suggesting in the midst of the convention that the U.S. might not defend America's NATO partners with him as president. The remarks, in an interview published online Wednesday by The New York Times, deviate from decades of American doctrine and seem to reject the 67-year-old alliance's bedrock principle of collective defence.

Donald Trump speaks on the last day of the Republican National Convention on July 21. (Photo: Jim Watson/AFP/Getty Images)

Trump reinforced his position from the convention stage, saying the United States has been "picking up the cost" of NATO's defences for too long. He also disavowed America's foreign policy posture under both Democratic and Republican presidents, criticizing "fifteen years of wars in the Middle East'' and declaring that "Americanism, not globalism, will be our credo."

"As long as we are led by politicians who will not put 'America First,' then we can be assured that other nations will not treat America with respect," he said.

"As long as we are led by politicians who will not put 'America First,' then we can be assured that other nations will not treat America with respect."

He had promised to describe "major, major" tax cuts. But his economic proposals Thursday night were vague, centring on unspecified plans to create millions of jobs. He promised a "simplified" tax system for the middle class and businesses, fewer regulations and renegotiation of trade deals that he says have put working class Americans at a disadvantage.

At every turn, Trump drew sharp contrasts with Clinton, casting her as both unqualified for the presidency and too tied to Washington elites to understand voters' struggles.

Democrats will formally nominate Clinton at their convention next week in Philadelphia. She is on the verge of naming a running mate to join her in taking on Trump and his vice-presidential pick, Indiana Gov. Mike Pence, in the general election. Virginia Sen. Tim Kaine has emerged as her top choice.